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This picture is an example of early autotrophs.
Click on image for full sizeImage courtesy of Corel Photography

Early Life

Organisms that are able to make their own food (in the form of sugars) by
using the energy of the
Sun are called autotrophs, meaning "self-feeders".
Photosynthesis is the name of the process through which these autotrophs covert solar energy to biological energy.

Because the autotrophic bacteria were no longer dependent on the same
limited food supply as their heterotrophic
ancestors, they were able to flourish. Photosynthesis was mostly along sulphur or methane pathways, however, and not the familiar carbon dioxide pathway most plants use today. Moreover, these photosynthetic pathways are not oxygenic (O2 is not produced). The following is an example:

As a result of the activity of these bacteria, S began to build up in the world's atmosphere and oceans. The production of atmospheric O2 became possible only later after the oxidation state of the oceans changed.

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